Galactic cosmic ray flux simulation and prediction.

A dynamic galactic cosmic ray model is proposed to quantitatively describe the z=1-28 ions and electrons of E=10-10(5) MeV/nucleon and their particle flux variations around the Earth's orbit and beyond the Earth's magnetosphere due to diverse large-scale variations of solar activity factors. The variations of large-scale heliospheric magnetic fields and the galactic cosmic ray flux variation time delays relative to solar activity variations are simulated. The lag characteristics and sunspot number predictions having been determined in detail, the model can be used to predict galactic cosmic ray flux levels.     

Galactic cosmic ray composition and energy spectra.

Galactic cosmic ray nuclei represent a significant risk to long-duration spaceflight outside the magnetosphere. We review briefly existing measurements of the composition and energy spectra of heavy cosmic ray nuclei, pointing out which species and energy ranges are most critical to assessing cosmic ray risks for spaceflight. Key data sets are identified and a table of cosmic ray abundances is presented for elements from H to Ni (Z = 1 to 28). Because of the 22-year nature of the solar modulation cycle, data from the approaching 1998 solar minimum is especially important to reducing uncertainties in the cosmic ray radiation hazard. It is recommended that efforts to model this hazard take advantage of approaches that have been developed to model the astrophysical aspects of cosmic rays.     

Galactic cosmic ray abundances and spectra behind defined shielding.

LET spectra have been measured for lunar missions and for several near Earth orbits ranging from 28 degrees to 83 degrees inclination. In some of the experiments the flux of GCR was determined separately from contributions caused by interactions in the detector material. Results of these experiments are compared to model calculations. The general agreement justifies the use of the model to calculate GCR fluxes. The magnitude of variations caused by solar modulation, geomagnetic shielding, and shielding by matter determined from calculated LET spectra is generally in agreement with experimental data. However, more detailed investigations show that there are some weak points in modeling solar modulation and shielding by material. These points are discussed in more detail.     

Galactic cosmic ray modulation for sunspot cycle 23

We present a study of the galactic cosmic ray modulation for sunspot cycle 23. We use the monthly and the annual mean hourly, pressure corrected, data from neutron monitors of the global network (monthly rate is calculated as the average of the hourly pressure corrected values). We draw attention to an asymmetry in the galactic cosmic ray (GCR) recovery during odd and even cycles for the monthly mean hourly rate data. For over half a century of observations, we find that the recovery for the odd cycles is to a higher level than for the even cycles. Qualitatively the effect is ascribed to charged particle drifts in inhomogeneous interplanetary magnetic field. Even so it has not been possible to arrive at a quantitative, self-consistent, explanation in terms of drifts at higher and lower GCR rigidities. We also study the rigidity dependence of the amplitude of 11-year modulation over a wide range (1–200 GV) of GCR spectrum; it is a power law in rigidity with an exponent −1.22. We discuss the implication of these findings on quasi-linear diffusion theories of modulation. We reflect on GCR recovery pattern for 2006–2009.     

Galactic cosmic ray radiation levels in spacecraft on interplanetary missions.

Using the Langley Research Center galactic cosmic ray (GCR) transport computer code (HZETRN) and the computerized anatomical man (CAM) model, crew radiation levels inside manned spacecraft on interplanetary missions are estimated. These radiation-level estimates include particle fluxes, LET (linear energy transfer) spectra, absorbed dose, and dose equivalent within various organs of interest in GCR protection studies. Changes in these radiation levels resulting from the use of various different types of shield materials are presented.     

Ground-based simulations of galactic cosmic ray fragmentation and transport.

Since mean free paths for nuclear fragmentation are of the order of the ranges of primary Galactic Cosmic Ray (GCR) nuclei, determination of the radiation field produced by successive fragmentations of nuclei in material and tissue is essential to accurate assessment of GCR radiation risk to humans on long-duration missions outside the geomagnetosphere. We describe some recent measurements made at the Bevalac of heavy ion transport through materials, with representative results and examples of how they may be applied to aspects of the space radiation problem, including efforts to devise analytical tools for predicting biological effects and for designing spacecraft shielding.     

Galactic cosmic radiation model and its applications.

A model for the differential energy spectra of galactic cosmic radiation as a function of solar activity is described. It is based on the standard diffusion-convection theory of solar modulation. Estimates of the modulation potential based on fitting this theory to observed spectral measurements from 1954 to 1989 are correlated to the Climax neutron counting rates and to the sunspot numbers at earlier times taking into account the polarity of the interplanetary magnetic field at the time of observations. These regression lines then provide a method for predicting the modulation at later times. The results of this model are quantitatively compared to a similar Moscow State University (MSU) model. These model cosmic ray spectra are used to predict the linear energy transfer spectra, differential energy spectra of light (charge < or = 2) ions, and single event upset rates in memory devices. These calculations are compared to observations made aboard the Space Shuttle.     

Solar modulation and nuclear fragmentation effects in galactic cosmic ray transport through shielding.

Crews of manned interplanetary missions may accumulate significant radiation exposures from the galactic cosmic ray (GCR) environment in space. Estimates of how these dose levels are affected by the assumed temporal and spatial variations in the composition of the GCR environment, and by the effects of the spacecraft and body self-shielding on the transported radiation fields are presented. In this work, the physical processes through which shielding alters the transported radiation fields are described. We then present estimates of the effects on model calculations of (1) nuclear fragmentation model uncertainties, (2) solar modulation, (3) variations between solar cycles, and (4) proposed changes to the quality factors which relate dose equivalent to absorbed dose.     

The transport of galactic and jovian cosmic ray electrons in the heliosphere

An overview is given on what we know about the cosmic ray diffusion process from the modelling of low-energy (MeV) electron transport in the heliosphere. For energies below ∼300 MeV, these electrons give a direct indication of the average mean free paths because they do not experience large adiabatic energy changes and their modulation is largely unaffected by global gradient and curvature drifts. Apart from galactic cosmic ray electrons, the jovian magnetosphere at ∼5 AU in the ecliptic plane is also a relatively strong source of MeV electrons, with energies up to ∼30 MeV. Therefore, when modelling the transport of these particles in the inner heliosphere, a three-dimensional treatment is essential. By comparing these models to observations from the Ulysses, Pioneer and Voyager missions, important conclusions can be made on e.g., the relative contributions of the galactic and jovian electrons to the total electron intensity, the magnitude of the parallel and perpendicular transport coefficients, and the time dependant treatment thereof.     

Galactic cosmic rays and cell-hit frequencies outside the magnetosphere.

An evaluation of the exposure of space travelers to galactic cosmic radiation outside the earth's magnetosphere is made by calculating fluences of high-energy primary and secondary particles with various charges traversing a sphere of area 100 microns2. Calculations relating to two shielding configurations are presented: the center of a spherical aluminum shell of thickness 1 g/cm2, and the center of a 4 g/cm2 thick aluminum spherical shell within which there is a 30 g/cm2 diameter spherical water phantom with the point of interest 5 g/cm2 from the surface. The area of 100 microns2 was chosen to simulate the nucleus of a cell in the body. The frequencies as a function of charge component in both shielding configurations reflects the odd-even disparity of the incident particle abundances. For a three-year mission, 33% of the cells in the more heavily shielded configuration would be hit by at least one particle with Z greater than 10. Six percent would be hit by at least two such particles. This emphasizes the importance of studying single high-Z particle effects both on cells which might be "at risk" for cancer induction and on critical neural cells or networks which might be vulnerable to inactivation by heavy charged particle tracks. Synergistic effects with the more numerous high-energy protons and helium ions cannot be ruled out. In terms of more conventional radiation risk assessment, the dose equivalent decreased by a factor of 2.85 from free space to that in the more heavily shielded configuration. Roughly half of this was due to the decrease in energy deposition (absorbed dose) and half to the decrease in biological effectiveness (quality factor).     

Gamma rays and cosmic ray gradiants

The advent of data from the Gamma Ray Observatory, of much higher precision than hitherto, will open up the possibility of deriving more precise results on the presence of ‘cosmic ray gradients’ in the Galaxy. There will still be a variety of difficulties, however. There are still uncertainties in the mass of gas in each of its major forms, HI, H₂ and HII, and their corresponding spatial distribution and the photon field in the Galaxy are known imprecisely. The problem of the contribution to the gamma ray flux from unresolved discrete sources will still be present, too, albeit at a lower level than previously. The division of parent particles between electrons and protons (nuclei) will also continue to be rather uncertain.

These problems are examined in the paper.     

Galactic cosmic ray gradients in the ecliptic plane and at high latitudes during two solar cycles (meteorite data)

Results of investigations of cosmogenic isotope radioactivity in chondrites fallen to earth during two solar cycles are presented. The data obtained on radial and latitudinal gradients cover the period 1955–1976, heliocentric distances from 1.03 AU to 3.33 AU, and heliographic latitudes from 23°S to 16°N. The dependence of radial and latitudinal gradients on the phase of solar activity is established, as well as a north-south asymmetry during a certain period after the inversion of the general solar magnetic field in 1969.     

Effect of random nature of cosmic ray sources – Supernova remnants – on cosmic ray intensity fluctuations,anisotropy, and electron energy spectrum

The random nature of sources (the supernova remnants) leads to the fluctuations of cosmic ray intensity in space and time. We calculate the expected fluctuations in a flat-halo diffusion model for particles with energies from 0.1 to 10³ TeV. The data on energy spectra and anisotropy of very high energy protons, nuclei and electrons, and the astronomical data on supernova remnants, the potential sources of cosmic rays, are used to constrain the value of the cosmic-ray diffusion coefficient and its dependence on energy.     

Gamma-ray astronomy and cosmic ray origin problem

We analyse the results of the nonlinear kinetic theory of cosmic ray (CR) acceleration in supernova remnants (SNRs) in order to describe their relevant properties: the remnant dynamics and the characteristics of nonthermal emission produced by CRs. It is shown that the theory fits the existing data in a satisfactory way and that the magnetic field in SNRs is significantly amplified due to efficient acceleration of the nuclear CR component. From the fact that magnetic field amplification occurs in all the young SNRs for which relevant data exist, and given the strong theoretical connection between magnetic field amplification and efficiently accelerating the nuclear CRs, we tentatively conclude that the Galactic SNRs are the source population of the Galactic CRs. Due to high interior magnetic fields in young SNRs the π⁰-decay γ-rays generated by the nuclear CR component as a rule dominate over γ-rays generated by the electron CR component, and the calculated γ-ray flux fits existing data.     

An analytical model, describing dynamics of galactic cosmic ray heavy particles.

The present paper analyses the problems of modeling galactic cosmic ray particle fluxes. A model representation which enables the particle energy spectra for large-scale solar activity induced modulations to be calculated is described.     

Radiation protection issues in galactic cosmic ray risk assessment.

Radiation protection involves the limitation of exposure to below threshold doses for direct (or deterministic) effects and a knowledge of the risk of stochastic effects after low doses. The principal stochastic risk associated with low dose rate galactic cosmic rays is the increased risk of cancer. Estimates of this risk depend on two factors (a) estimates of cancer risk for low-LET radiation and (b) values of the appropriate radiation weighting factors, WR, for the high-LET radiations of galactic cosmic rays. Both factors are subject to considerable uncertainty. The low-LET cancer risk derived from the late effects of the atomic bombs is vulnerable to a number of uncertainties including especially that from projection in time, and from extrapolation from high to low dose rate. Nevertheless, recent low dose studies of workers and others tend to confirm these estimates. WR, relies on biological effects studied mainly in non-human systems. Additional laboratory studies could reduce the uncertainties in WR and thus produce a more confident estimate of the overall risk of galactic cosmic rays.     

Ground-based measurements of galactic cosmic ray fragmentation in shielding.

The mean free path for nuclear interactions of galactic cosmic-rays is comparable to shielding and tissue thicknesses present in human interplanetary exploration, resulting in a significant fraction of nuclear reaction products at depth. In order to characterize the radiation field, the energy spectrum, the angular distribution, and the multiplicity of each type of secondary particles must also be known as a function of depth. Reactions can take place anywhere in a thick absorber; therefore, it is necessary to know these quantities as a function of particle energy for all particles produced. HZE transport methods are used to predict the radiation field; they are dependent on models of the interaction of man-made systems with the space environment to an even greater extent than methods used for other types of radiation. Hence, there is a major need to validate these transport codes by comparison with experimental data. The most cost-effective method of validation is a comparison with ground-based experimental measurements. A research program to provide such validation measurements using neon, iron and other accelerated heavy ion beams will be discussed and illustrated using results from ongoing experiments and their comparison with current transport codes. The extent to which physical measurements yield radiobiological predictions will be discussed.     

Modeling ground and space based cosmic ray observations

After entering our local astrosphere (called the heliosphere), galactic cosmic rays, as charged particles, are affected by the Sun’s turbulent magnetic field. This causes their intensities to decrease towards the inner heliosphere, a process referred to as modulation. Over the years, cosmic ray modulation has been studied extensively at Earth, utilizing both ground and space based observations. Moreover, modelling cosmic ray modulation and comparing results with observations, insight can be gained into the transport of these particles, as well as offering explanations for observed features. We review some of the most prominent cosmic ray observations made near Earth, how these observations can be modelled and what main insights are gained from this modelling approach. Furthermore, a discussion on drifts, as one of the main modulation processes, are given as well as how drift effects manifest in near Earth observations. We conclude by discussing the contemporary challenges, fuelled by observations, which are presently being investigated. A main challenge is explaining observations made during the past unusual solar minimum.     

Comparison of measured cosmic ray LET spectra with models and predictions.

Radiation effects of cosmic ray nuclei are generally described as a function of the particle LET. For a large number of space missions LET spectra have been measured and models have been developed to calculate these spectra that include the effects of geomagnetic shielding and shielding provided by material. In this paper we compare measured and calculated LET spectra. For low earth orbits events with high local energy deposition, i.e., short range secondaries, contribute significantly to the measured spectra. These events are produced by nuclear interactions, mainly induced by protons from the south atlantic anomaly. The technique to include these contributions in the models depends on the size of radiation sensitive volumes. For sizes comparable to or larger than the range of target secondaries it is essential to separate contributions by target interactions from those of cosmic rays. This separation is possible in experiments which use stacks of plastic nuclear track detectors. The yield of short range events generated by protons and measured in the detector can be calibrated from accelerator experimental data. We present first results for CR-39 detectors.